1. Field of the Invention
This invention relates to the dispersal of a multi-component liquids, such as insecticides and fragrance solutions, by ejecting a cloud or mist of small droplets of the liquid from a vibrating atomization plate into the atmosphere and evaporating the liquid components from the droplets as they fall back through the atmosphere.
2. Description of the Related Art
It is well known to disperse fragrances and insecticides into the atmosphere by using a vibrating atomization plate to form a mist or cloud of small droplets of a solution containing the fragrance or insecticide and to eject the mist or cloud into the atmosphere in the form of minute liquid droplets. As the mist or cloud settles, the fragrance or insecticide evaporates from the droplets. Examples of devices for doing this are shown in U.S. Pat. No. 4,085,893, U.S. Pat. No. 5,173,274, U.S. Pat. No. 5,601,235 and U.S. Pat. No. 5,894,001. In general these devices supply the liquid fragrance or insecticide to a vibrating atomization plate which, due to its vibrations, breaks up the liquid into fine droplets and ejects them upwardly in the form of a mist or a cloud. As the droplets fall back down, the fragrance or insecticide evaporates from the droplets and disperses into the atmosphere.
A problem occurs in the operation of these known devices in that there is no means to be sure that all of the liquid which is ejected will evaporate before the droplets fall back onto surrounding surfaces. As a result, an unsightly and often destructive liquid residue of unevaporated liquid builds up on these surfaces. This problem is particularly difficult where the liquid to be ejected is a fragrance or an insecticide. This is because fragrance and insecticide compositions are generally quite complex; and there has been no way to know in advance that a particular composition will fully evaporate when subjected to atomization in a vibrating plate atomizer.
The present invention substantially overcomes the problem of undesired buildup of unevaporated liquid which has been ejected from a vibrating plate atomizer as a mist or cloud into the atmosphere. The invention is based on the discovery that when liquid compositions are broken up into small droplets and ejected into the atmosphere above a surrounding surface, such as a table top, for example, the ability of those droplets to become fully evaporated before they fall back onto the surrounding surface, does not depend on the vapor pressure of the liquid composition itself. Instead the ability of the droplets to evaporate depends upon the vapor pressures of the individual components of the liquid composition. The invention is also based on the discovery that the vapor pressure of the lowest vapor pressure component of the liquid composition must be such that this component will evaporate before the liquid droplet which contains the component reaches the surrounding surface.
According to one aspect of the invention, there is provided a novel method of evaporating a multi-component liquid solution, particularly, a multi-component liquid fragrance or a multi-component liquid insecticide in a manner such that the amount of liquid deposited on adjacent surfaces is minimized. This novel method involves the steps of using a vibrating plate atomizer to form a mist or cloud of small liquid droplets of the solution, ejecting the mist or cloud into the atmosphere and allowing the droplets to fall back toward an adjacent surface. The liquid solution comprises a plurality of components having respective vapor pressures; and the component having the lowest vapor pressure is related to those droplets having the larger diameters such that
1.2xc3x971012xc3x97Dp4/[Hxc3x97Pv]xe2x89xa61 
where Dp is the diameter, in centimeters, of the large diameter droplets, H is substantially the height, in centimeters, to which the large diameter droplets are ejected above the adjacent surface, and Pv is the vapor pressure, in millimeters of Hg, of those components having the lowest vapor pressure. In this manner, the amount of unevaporated liquid that falls back upon the adjacent surface is minimized. In more specific aspects, the values Dp, H and Pv are chosen in conjunction with the effects of the liquid on the adjacent surface such that any unevaporated liquid that does fall back onto the adjacent surface will be in an amount insufficient to cause an adverse effect on the surface.
According to another aspect of the invention, there is provided a novel apparatus for evaporating a multi-component solution, particularly a multi-component liquid fragrance or a multi-component liquid insecticide. This novel apparatus comprises a liquid reservoir containing a multi-component liquid, a liquid delivery system and a vibrating plate atomizer. The liquid delivery system is arranged to convey liquid from the reservoir to the atomizer. The atomizer in turn is constructed to form a mist or cloud of small liquid droplets and to eject these droplets into the atmosphere. The droplets have diameters within a predetermined range, and they are ejected to predetermined heights above an adjacent surface. As the droplets fall back toward the surface they evaporate to an extent that no or only a negligible amount of liquid falls upon the adjacent surface. The liquid comprises a plurality of components having respective vapor pressures; and the component having the lowest vapor pressure is related to those droplets having the larger diameters such that
1.2xc3x971012xc3x97Dp4/[Hxc3x97Pv]xe2x89xa61 
where Dp is the diameter, in centimeters, of the larger diameter droplets, H is the height in centimeters, to which the larger diameter droplets are ejected and Pv is the vapor pressure, in millimeters of Hg, of those liquid components having the lowest vapor pressure. In this manner the amount of unevaporated liquid that falls back upon the adjacent surface is minimized. In more specific aspects, the values Dp, H and Pv are chosen in conjunction with the effects of the liquid on the adjacent surface such that any unevaporated liquid that does fall back onto the adjacent surface will be in an amount insufficient to cause an adverse effect on the surface.
In a still further aspect, the present invention is based on the discovery that the evaporation of an atomized droplet can be predicted by the evaporation rate of a pendant drop of the liquid by Axisymmetric Drop Shape Analysis (ADSA). In this technique, a drop of known size is formed at the end of a capillary. Then, while the drop is exposed to the atmosphere, measurements are taken of the rate at which its size decreases. If the rate at which the size of the pendant drop decreases is greater than a given threshold, then the liquid is suitable for use in an atomizer. That is, evaporation of an atomized droplet from the atomizer will be sufficiently complete before the droplet can reach the adjacent surface. The given threshold is chosen such that for the diameter of the droplets comprising the mist or cloud, and the height to which they are ejected above the adjacent surface, substantially all of the liquid will evaporate before the droplets fall back onto the adjacent surface.
In a more specific aspect, the threshold is established by measuring the rate at which the size of a pendant drop of the liquid decreases. This corresponds to the rate of evaporation of liquid from the pendant drop. In the case of a multi-component liquid fragrance or a multi-component liquid insecticide where the pendant drop of the liquid is about 6 microliters, the evaporation rate should be measured when approximately 70% of the volume of the drop has evaporated.
Other specific aspects of the invention involve novel apparatus for ascertaining the rate at which a test pendant drop of a liquid decreases in size, and methods and apparatus for ascertaining evaporability based on measurements of surface tension of a droplet of the liquid.